The Fourdrinier process of paper making involves a succession of phases. Initially a slurry of cellulose fibers in water is distributed on a screen and some of the water is drained off. A web is formed which is then transported by a felt or a succession of felts to pass a number of nip rollers in a press section. The felt and the formed web are squeezed between the nip rollers to extract water mechanically. In current practice, the web leaving the press section contains from 35 to 45% solids. The web then passes through a dryer section consisting of heated cylinders, in which the water content of the web is reduced by evaporation to roughly that of the finished paper.
Size coaters often follow the dryer section, followed by afterdryers and calenders, ending with the reel. The dryers and afterdryer sections may contain 60 or more heated cylinders. A felt is used to hold the paper firmly against many of the heated dryer cylinders, for assuring contact of the web with the heated surface and thereby promoting drying efficiency. Drying the web is the result of evaporation, caused by conduction of heat from the cylinders into the fibrous moisture-laden web. The term moistureladen refers to water in all forms carried by the web, as free water or as moisture bound to the web's fibers.
In the U.S., roughly half the production is paperboard, which is formed into substantially thicker and heavier sheets than paper and newsprint. Many paperboard machines do not use papermaker's felts in the final dryer sections, because they are not necessary.
When the cold web enters the dryer section, fibers may be picked out of the web, adhering to the hot dryer cylinders. To suppress that effect, the temperature of the first series of dryer cylinders is comparatively low. Each successive cylinder's temperature is progressively higher until the sheet has been warmed up sufficiently for the web to encounter a hot dryer cylinder without concern for "picking" of fibers.
The following series of dryer cylinders effect a constant rate of drying. In this region the cylinders' temperature may be uniform. The paper making machine includes a falling rate zone that follows after the constant rate zone. The temperature of the steam in the successive cylinders of the falling rate zone is increased to 370.degree. F. (187.degree. C.). This is the practical upper limit for cylinders heated by steam under pressure. In the falling rate zone, the rate of evaporation declines progressively, due to the relatively dry condition of the web; in that condition, the web is a poor heal, conductor, so that the transfer of heat to the web declines.
The highest pressure steam is typically delivered to the final dryer section, and a cascade steam system delivers reduced temperature steam upstream, to each cylinder of the series of dryer cylinders. It is complicated and expensive to provide steam at a pressure such that a specified high temperature is maintained in each of the cylinders. This is especially true when temperature changes are to be made.
Steam-heated cylinders are massive, both because of their large size and substantial wall thickness. They are usually made of gray cast iron for economy, and their walls are quite thick; e.g., 1" to 2" (25mm. to 51 mm.) or more, to withstand the high internal steam pressure. A web may be 25 ft. (7.6 m.) wide, requiring cylinders that are slightly longer. The web may travel at 3300 ft./min. (1000 m./min.), or roughly 37 miles/hr. (60 km./hr.). That speed is impressive. The dryer section typically includes 60 cylinders. By any standard, the capital investment in a paper making machine is huge, and a considerable amount of space is needed.
Various types of paper making apparatus differ from that outlined above. For example, the "Yankee" type is characterized by inclusion of one very large diameter dryer cylinder; e.g., a diameter of 12 ft. to 18 ft. (3.6 m. to 5.5 m.). There, the wall thickness is particularly great, to withstand the pressure of the contained high-temperature steam and to allow for periodic grinding to restore surface smoothness.
The highest temperature of any steam-heated cylinder is limited by the corresponding pressure of steam that can be safely contained within the cylinder. The maximum internal steam temperature of a dryer cylinder (see above) is approximately 370.degree. F. (187.degree. C.) because of concern for the high steam pressure. It has been widely recognized that higher regulated temperatures, if feasible, would accelerate the drying process and would reduce substantially the number of dryer cylinders required.
Paper machine drying sections, worldwide, are almost universally heated by steam under pressure. Accordingly, it is appropriate to consider such apparatus in further detail, as a basis for appraising the advance in the art represented by the present invention.
As noted above, the temperature of a drying cylinder in a paper making machine is not determined by that which would be desirable from the point of view of performance, but by the limitations of cylinders heated by steam to withstand high pressures safely. This is evidenced by the large numbers of drying cylinders required in high-speed paper making machines or by the limited machine speed with lower temperature cylinders performing the drying function. Cylinders heated by steam under pressure have other significant limitations.
The external surface of a steam-heated cylinder responds slowly to an adjustment in steam pressure. This slow response time is manifested, for example, by the many minutes needed to bring the paper making machine from a cold start to full-speed operation. It is also manifested by the delayed change of a cylinder's external temperature in response to an adjustment in steam pressure.
It is virtually impossible to regulate the temperature of a cylinder wall from point-to-point along its length, for developing a desired temperature profile across the width of a web being dried. It is well-known that steam cylinder dryers are hotter at the ends, where no moist paper is present to absorb thermal energy from the cylindrical shell and from the end walls of the cylinder. Complicated, cumbersome arrangements have been proposed in an effort to compensate for the otherwise excessive cylinder temperatures at the margins of the web. These have been intended to control edge curl caused by unrestrained and excessive drying at the edges of the sheet. However, no easy, practical way has been found for varying the cross-machine temperature profile of a cylinder heated by steam under pressure.
The cross-machine moisture profile of a web emerging from the main dryer in a machine for producing paper and paperboard tends to develop non-uniformity not only at the margins but also at other portions of its width. This results from cumulative effects in the forming, press, and dryer sections. A web with moisture streaks is poorly suited to being coated as with size; moisture variations of the web cause the coating to be non-uniform. Also, a web whose cross-machine moisture profile is non-uniform has a tendency to render the calendering non-uniform.
The foregoing and other characteristics of a machine for making paper or paperboard, having dryer cylinders heated by steam under pressure, are impaired by some of the traits of the cylinder wall. Transfer of heat from the steam to the outer surface of the cylinder which contacts the web is impeded by many factors, including:
a) The considerable thickness of the cylinder wall needed for containing steam under the high pressure corresponding to the steam temperature, noting that the actual wall thickness is greater by a safety factor of 2.8 times that theoretically required for withstanding the steam pressure; PA1 b) The poor heat conductivity of gray cast iron, the customary metal chosen for the cylinder wall, rather than a more expensive metal of superior heat conductivity; PA1 c) A layer of condensate that forms and is distributed by centrifugal force over the cylinder's interior; PA1 d) A layer of scale that develops over the cylinder's internal surface; and PA1 e) A temperature drop required to extract heat from the steam, by condensation.
The difference between the temperature of the steam and that of the cylinder's external surface represents a waste of energy.
The enormous mass of the cylinder wall and the high inertial load require a large value of installed horsepower capacity and a correspondingly high energy cost to drive the machine.
The above factors that impede energy transfer, plus the thermal inertia of the massive cylinder wall, contribute to a long response time of steam-heated cylinders. The same factors limit the speed and productivity of the machine.
In emergencies such as web breaks, the drying process is upset and the steam valves often fail to respond quickly, filling dryers with varying levels of condensate. The large amount of thermal inertia of the heavy-walled steam-heated cast iron cylinders imposes a long time delay should the dryers require maintenance or clearing.
Recognition of the problems and limitations of steam as the heat source in dryers of paper making machines has prompted proposals of alternative heating media.
It has been proposed that dryer cylinders in paper making machines should be heated internally by electric power; but electricity is inordinately expensive.
It has also been proposed that a dryer cylinder for paper making apparatus should be heated by a flame within the cylinder. Transfer of heat from the gaseous combustion products to the cylinder requires extensive areas of metal exposed to the hot gases and requires efficient removal of the combustion products after their heat has been extracted, so as to provide necessary space that is to receive newly emitted gaseous exhaust. See Hemsath et al., U.S. Pat. No. 4,693,015 issued Sept. 15, 1987, Calhoun U.S. Pat. No. 2,987,305 issued Jun. 6, 1961, and Bourrel et al., U.S. Pat. No. 3,729,180.
U.S. Pat. No. 4,688,335 issued Aug. 25, 1987 to Krill et al. discloses use of a gas-fired radiant heat generator to heat a cylinder that acts on a web of fibers being pressed against the cylinder by a felt and a nip roller, the web having a large water content. The burner of Kill et al. is in the form of a ceramic fiber matrix shaped as a cylindrical shell. The cylinder's fiber matrix is to heat the cylindrical shell uniformly about its entire periphery. An air-fuel mixture is supplied to the interior of the shell. The mixture burns as it emerges everywhere from the shell. Unlike Hemsath, above, the energy of combustion in Krill et al. is intended to produce radiant heat. The heated web-engaging cylinder in Krill et al. operates at 600.degree. F. to 800.degree. F. (315.degree. C. to 427.degree. C.). That heat is so intense that some of the free water that is present in spaces between the fibers of the web is converted to steam, which blasts other free water through end out of the web. This process is called "impulse drying". Even though the supplied air-fuel mixture is adjustable, reduction of the air-fuel supply is limited by the lowest rate needed to sustain combustion. Noting that the type of burner used in Krill at al to produce radiant heat is in the form of a complete cylinder, the heat output would almost certainly be excessive for use in the usual drying section of a paper-making machine, even with its air-fuel supply adjusted downward to a minimum. Moreover, if the temperature of the cylinder were reduced by adjusting the supply of air-fuel mixture for developing a suitable operating temperature at full-speed operation of the apparatus, little if any latitude of downward adjustment would be available for realizing still lower cylinder temperatures as is required during slowed operation of the apparatus.
An earlier form of impulse dryer is acknowledged by Krill et al., referred to in U.S. Pat. No. 4,324,613 issued to Wahren. An external IR burner is used in Wahren to heat an arc of a cylinder's exterior to a high temperature. The newly formed web with its high moisture content is subjected to intense pressure between nip rollers and the just-heated segment surface of the cylinder to induce impulse drying. The hot segment of the cylinder's surface is chilled promptly in this process; it is reheated by the external IR burner during on-going rotation. The cylinder's exterior is a poor heat conductor, to avoid temperature-reducing conduction of heat away from the heated surface, thereby to conserve the heat for transfer to the moisture-laden web.
In usual machines for making paper or paperboard, the moist web of fibers is dried by evaporation. The web is constrained against a large surface area of each of many steam-heated cylinders in succession. Despite alternatives that have been proposed for heating the dryer cylinders of apparatus for making paper and paperboard, steam under pressure continues to be the generally accepted heating medium.